The Papaver L. plant have been well known as a source of pharmaceutically valuable alkaloids (noscapine, thebaine, codeine, roemerine, papaverine and morphine). The current study investigates the phytochemical, in-vitro antioxidant, and anticancer activities of papaver decaisnei, an endemic plant species to the flora of Kurdistan-Iraq. The chemical analysis of the methanolic (MeOH) extracts of flowers, leaves, and roots of papaver decaisnei were made by using gas chromatography-mass spectrophotometry (GC-MS), and the antioxidant activity evaluation done by radical scavenging [on 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2jazino-bis (3- ethylbenzothiazoline-6-sulfonic acid) (ABTS)], and reducing power [cupric reducing antioxidant capacity (CUPRAC), and ferric reducing antioxidant power (FRAP)] assays. The anticancer actions were presented as IC50 (inhibitory concentration at 50%) on human colorectal adenocarcinoma (Caco-2), mammary cancer cells (MCF-7), and human cervical carcinoma (HeLa) cells. The results of the phytochemical analysis showed 17, 19, and 22 chemical compounds for flowers, leaves, and roots of P. decaisnei, respectively. The prevalent organic compounds of P. decaisnei were alkaloids, phenolics, fatty acids, esters, and phytosterols, namely Roemerine (70.44%), Decarbomethoxytabersonine, 9,12,15- Octadecatrien-1-ol, Hexadecanoic acid, 6,8-Dioxa-3-thiabicyclo(3,2,1)octane 3,3-dioxide, and γ-Sitosterol. The antioxidant activity of plant organ extracts was within 39.1-143.5 μg/ml for DPPH and 123.12-276.4 μg/ml for ABTS assays, while, the FRAP and CUPRAC values ranged within 12.4- 34.3 and 42.6-75.8 μg/ml, respectively. The anticancer action of P.decaisnei organ extracts was found against all tested human cell lines (Caco-2, MCF-7, HeLa) with inhibitory concentrations (IC50 ) values between 125.3-388.4 μg/ml. The presented data on alkaloid contents and biological activity of P. decaisnei can serve a ground knowledge for the future biomedical synthesis and cancer research projects.